Data hiding is the process of encoding extra (“steganographic”) information in digital data, such as in images, by making small modifications to the data. Hidden information in images may be used to supplement an image with additional information, or to verify some aspect of the image, such as origin or the integrity of the image.
Technology for producing images which contain steganographic information, such as in the form of digital watermarks, is known. Commercial products which can store and read digital watermarks are widely available. The embedded data typically remains with the image when it is stored or transmitted. The embedded data may be meant to be extracted by an end user, or hidden to the end user. Some of these techniques (e.g., glyphs or bar codes) use areas outside of the image area because the techniques are largely destructive of the image itself and require a uniform background to read the information.
It is also known to print patterns in different colors on a substrate such that the patterns may be viewed through one or more filters having certain correlated colors, such that the patterns will change, depending upon the colors involved. It is also known to print characters in different colors in an overlapping relationship such that the overlapped characters, when viewed through one colored filter, will give the appearance of only certain ones of the superimposed characters, and when viewed through a second and differing colored filter, will reveal certain other ones of the superimposed characters. Such approaches are known for encoding information to prevent recognition of the information content of the pattern until the pattern is decoded and made comprehensible. These approaches have been applied to promotional gaming technology and in document security and document verification applications.
Means have been used to physically alter a secure document so as to subsequently enable the marked document to be authenticated: by the use of special fibers, threads, chips, fluorescent platelets and particles, and fluorescent or phosphorescent inks, for example. Other techniques such as the adhesion of logos, labels, magnetic strips, or stickers to an image are employed, but these can be easily removed or hidden, and these devices can also be unsightly. Most of such approaches require that a specially modified scanning device be employed, such as by inclusion of a magnetic detector, for recovery of hidden information.
Methods are known for printing documents in a way to thwart unauthorized copying by use of modern color copiers. Most of them are based on altering the background of the original in order that the contrast may be reduced. Since the human eye and the scanner in the copier have different sensitivities for lightness values of colors, it has been attempted to darken the background during copying to a relatively larger extent for the sensor than for the eye.
There is a need for a system for the production of printed images on substrates, such as secure documents, that may be authenticated by an automated detection process. Such images should be secure against unauthorized reproduction or origination (e.g., substantially adverse to counterfeiting), be readily identifiable in the automated detection process, and substantially tamper-proof with respect to alteration of its embedded information.